Know-How Notes: How To Service Your Differential

Changing your engine oil and topping off fluids is a common procedure that many DIYers take care of in their driveway. These simple tasks are part of owning a car, but there is another service that you need to be mindful not to forget – servicing your rear differential. This is most commonly an issue for trucks, which are rear-wheel drive (or 4-wheel drive), but there are plenty of cars on the road with rear differentials too. Knowing when and how to service your rear axle is important and far too often overlooked.

The rear axle housing supports the axles and differential carrier, which transfer the power from the engine to the rear wheels. There are not very many moving parts, but they are all critical. Every rear end contains two wheel bearings, two differential bearings, two pinion bearings, three seals, a set of gears, two axles, and the differential itself. All of these items must have proper lubrication. If any of these fail, you have a major problem on your hands. Over time, the lubrication, known as gear oil, breaks down, losing its viscosity and protection.

The typical change interval for differential oil is between 100k, and 150k miles for normal, low stress use. As soon as you hook a boat or trailer to your car or truck, that change interval drops drastically. In fact, a vehicle that regularly pulls a trailer should get fresh gear oil in the axle ever 30k miles. That is five times more often than normal. That goes to show you how stressful pulling a trailer is on the rear axle. For cars that don’t pull trailers, hard driving and high speeds result in similar problems. Heat is the biggest factor here, as differentials do not have pumps or coolers, they rely solely on the oil sloshing around. As the oil thins out, it may slosh more, but the protection barrier isn’t there. This means more metal on metal contact and faster wear. The kicker is that you will never know you have a problem until it is too late.

Using synthetic gear oil is one way to increase the life of your rear axle. Synthetic oil is less susceptible to heat breakdown and loss of viscosity. It is still a good idea to follow the proper change intervals, but you are less likely to have damage with synthetic oils. There are a couple of important caveats when it comes to gear oil, however, as the type of differential you have requires different oil. The main thing to know is whether or not you have a clutch-style differential. Open carriers, the most common on older RWD vehicles, do not have any sort of positive traction device.

If your rear wheels rotate opposite of each other with the tires off the ground, then you have an open carrier. This does not necessarily work for newer vehicles, particularly truck built from the mid-1990s on up. These trucks often have what a mechanical locking differential (nicknamed “gov lock” by some), which acts like an open carrier until there is sufficient wheel slip to activate the swing-arm locker, which engages the clutch-style limited slip differential. So you may think it is an open carrier, but it isn’t. The reason this is important to know is that clutch-style LSDs (Limited Slip Differentials) require a special additive in the gear oil to protect the clutches. Some gear oils already have this mixed in, but most do not, you have to add it.

Changing The Fluid

Changing the fluid is a very simple process, but it is a little more involved than an oil change. Most vehicles require the rear differential cover be removed, though some do have an actual drain plug. You need the following to perform this project.

Sockets and 3/8” ratchet

Drain pan

Gasket scraper

Gasket and/or silicone gasket maker

Gear oil

LSD additive if required

If you are working on a truck, you likely do not need to lift the vehicle, however if you are working on a car, you need to raise the vehicle so that you can access the rear axle cover. Make sure that you support the vehicle with a pair of jacks stands and not just a jack. This is very important. Don’t take the risk. Place the drain pan under the diff cover and remove all of the bolts. Don’t worry about leaks, the cover will be stuck in place. If your housing has a drain plug, you can use it instead of removing the cover.

Using a pry bar, flat blade screwdriver or a gasket scraper, carefully pry open the bottom of the cover. You may want to leave one top bolt lightly threaded in place so the cover doesn’t drop. Drain all of the fluid, watching for sparkles. If you have a lot of glitter in your oil, there is already damage, and you will need to have a rebuild. Unlike engine oil or transmission fluid, there is no smell test for gear oil. This is the nastiest smelling stuff on the earth when it is new, and it is even worse when used. Try to wear old clothes, as it stains and will stink for eternity.

Once the fluid is drained, fully remove the cover and clean the cover and inside of the housing with brake cleaner and a rag. You want to get as much of the old stuff out as possible. A small amount of glitter may be visible and for the first change, this is normal. If there is a lot, then you may need to have your rear axle checked out by an expert at your local NAPA AutoCare Service Center.

Use the scraper or a razor blade to remove all of the old gasket material from the cover. Clean it again with brake cleaner. If there is not a factory-installed magnet inside the cover, get one and put it on inside of the cover.

If using a gasket, place a few dabs of silicone gasket maker on the cover and then place the gasket on the cover. If you are just using gasket maker, place a nice 1/4” bead all the way around the cover, being sure to go around the inside edge of the bolt holes.

Carefully place the cover back onto the housing and reinstall the bolts in a criss-cross fashion, first hand-tight, then a little tighter, and then torque to the factory torque specs, which is typically 30 ft lbs.

On most housings, the filler hole is on the side of the center section, about half way up. This is almost always a 3/8” square drive plug, meaning you use the ratchet without a socket. Sometimes you need an extension to reach it. Remove the plug.

Filling the housing can be a bit of a pain, as it is tough to fit a bottle in the port. If you can, just stick the nozzle in the hole and squeeze the bottle until it is empty and move on to the next. You will end up with some left over in the bottle. After two bottles, combine the remaining fluid into one bottle. Most housings require 2.5-3 one-quart bottles of gear oil. If you can’t make the bottle fit, attach a piece of hose to the end, stick it in the port and hold the bottle outside of the car.

When the fluid trickles out of the port, it is full. Replace the plug.

That is all there is to servicing your rear end housing. It is fairly simple and you can do the job in your driveway. Make sure you clean up the tools and work area. Pour the old fluid into the empty bottles and dispose of properly. If you have any concerns about your vehicle, discuss it with your local NAPA AutoCare Service Center.

Check out all the drivetrain parts available on NAPA Online or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on how to service your differential, chat with a knowledgeable expert at your local NAPA AUTO PARTS store.

Photo courtesy of Wikimedia Commons.